1. Field of the Invention
This invention relates to mounting planar spectral filters to planar optical sensors and more specifically to a mounting structure and method using beads of epoxy bonded around the edge of the filter to the face of the sensor, and in certain configurations spacer balls that form standoff structures between the sensor and filter.
2. Description of the Related Art
A planar optical sensor includes a detector layer configured to detect incident light in an optically active area by converting photons to electrical charge, a readout integrated circuit (ROIC) that lies below the detector layer, and an array of cold-welded metal interconnects that connect the ROIC to the detector layer. The ROIC is configured to read out electrical charge through each interconnect corresponding to different regions on the detector layer to form a pixilated analog or digital signal.
The array of cold-welded metal interconnects is formed by first placing small bumps of soft metal, typically indium, at precise locations on each of the ROIC and the detector layer, precisely aligning the detector layer to the ROIC so that the bumps align and pressing the detector layer onto the ROIC to “cold fuse” the bumps to form a metal interconnect. This “hybridization” process is well established in the optical sensor industry for mounting the detector layer to the ROIC but does require expensive equipment and processes. The hybridization process is subject to variation due to compression force, number and size of indium bumps, and the volume of the indium bumps but is sufficient to provide reliable metal interconnects.
An optical sensor assembly also includes a spectral filter that passes one or more wavelength bands of interest. The spectral filter may pass a single wide band or multiple narrow bands. Individual bands or “zones” may be spatially registered to different x,y pixels or groups of pixel on the detector layer/ROIC. Placement of the filter in close proximity (e.g. 1 um to 350 um microns are typical depending on wavelength) to the sensor is critical to the sensor's optical performance. The spacing or “gap” in the z direction must be precisely and uniformly controlled across the optically active area.
A mounting structure should provide the required alignment fidelity in both the x,y directions in plane and z direction out of plane. The mounting structure must be sufficiently strong to withstand sheer stresses on the assembly, and should not transfer those stressed between the filter and sensor. The mounting structure should not introduce any structure into the optical path between the filter and the detector active area or contaminate the active area. The process for mounting the filter onto the assembly should not be prohibitively expensive.
The industry standard is to mount the filter to a bezel using epoxy and then mount the bezel onto the optical sensor assembly using mechanical fasteners. The bezel mounting configurations have increased volumetric size due to the added structure of the bezel. In addition, the bezel structure and its fasteners increase the system mass. This increase in mass requires adding additional features to the detector baseplate to achieve a robust and stiff design needed to overcome vibration.
Another approach is to place a thin layer of a curable, transparent adhesive such as an epoxy between the filter and the detector layer. The components are pressed together with the aid of appropriate tooling to a desired spacing prior to curing of the adhesive. After curing, the components are permanently bonded together with the layer of adhesive.
As described in U.S. Pat. Nos. 5,689,106 and 5,734,156, while operable, this fabrication technique has drawbacks. The adhesive bonding approach has relatively loose assembly tolerances in both the direction lying in the plane of the components and in the spacing between the components. The assembly time is relatively long, on the order of 12 hours, due to the need to at least partially cure the adhesive in the tooling. In operation of the device, the light reaching the sensor must pass through the adhesive layer, which attenuates and possibly distorts the light. The contact of the cured adhesive to the faces of the optical components can adversely affect their service lives.
U.S. Pat. Nos. 5,689,106 and 5,734,156 disclose using the hybridization process used to connect the detector layer to the ROIC, or a variant thereof, to join the filter to the optical sensor. The standoff structures comprise bonding elements (e.g. deformable metal bumps such as indium) on each of the filter and the optical sensor (outside the active area) that are cold-welded to form the structure. A standoff such as formed from silicon may be placed between the bonding elements. Alignment of the x/y position is achieved by precision placement and alignment of the bonding elements. Alignment of the z position is achieved by the metal standoff structure.
The hybridization approach provides an advance in the art of fabrication and assembly of optical devices in which two components in an optical train must be bonded together. The assembly cost is reduced due to a reduction in the bonding time. Tolerances are improved due to the stability of a metal bonding element as compared with a cured adhesive, which can distort as it cures. Optical performance is improved because no adhesive lies in the optical path, and the components are separated only by a gap. Long-term stability of the device is improved because no adhesive touches the faces of either of the components.